The present invention relates to a process cartridge for use in an image forming apparatus such as an electrophotographic copying apparatus, a laser printer, etc., in which at least a photosensitive member and a corona charger are integrally supported.
Of recent years, some compact copying apparatuses have employed a process cartridge in which a photosensitive member and a plurality of process devices, disposed around the photosensitive member, for example, a corona charger, a developing device, etc., are integrally mounted. In the process cartridge, the photosensitive member, the corona charger, a roller of the developing device, etc., are mounted in a casing made of plastic, so that the number of components of the known copying apparatuses is reduced and assembly of the known copying apparatuses can be performed easily. Meanwhile, in the corona charger, a shield casing is required to be electrically conductive, while a support portion for supporting a discharge wire should be electrically insulative so as to insulate the support portion from the shield casing. To this end, the shield casing is made of a metallic material or the like separately from the process cartridge and is mounted on the process cartridge by using machine screws, etc.
Meanwhile, in the case where a scorotron type corona charger is employed in the known process cartridge, a screen grid in which a metal plate of about 0.1 to 0.2 mm in thickness is formed with meshes provided at an opening of the corona charger. When a potential difference is produced between the screen grid and the shield casing, the screen grid and the shield casing are required to be insulated from each other. Therefore, as shown in FIG. 1, a screen grid 52 is attached to a pair of electrically insulative support portions 53 for supporting a discharge wire so as to be insulated from a shield casing 51.
However, the known process cartridge has such a drawback that in the case where the shield casing is provided separately from the process cartridge so as to be mounted on the process cartridge, the distance between the shield casing and the photosensitive member, which exerts an influence upon charging characteristics of the photosensitive member, greatly varies according to accuracy of assembly of the process cartridge. Thus, in order, to form the shield casing integrally with the casing, it has been considered that the casing be molded of electrically conductive resin, as proposed in Japanese Patent Laid-Open Publication No. 54-11747. However, in this case, since the support portion for supporting the discharge wire, which should be insulated from the shield casing, is required to be formed separately from the casing, so that accuracy of mounting of the support portion on the casing, namely, accuracy of distance between the discharge wire and the photosensitive member, poses a problem.
Meanwhile, the scorotron type corona charger of FIG. 1 is disadvantageous in that since the screen grid 52 is supported at its opposite end portions by only the support, portions 53, the screen grid 52 readily slackens. The screen grid 52 is formed by a thin metal plate having low rigidity and therefore, is likely to slackened. Once the screen grid 52 has been slackened, such a problem arises that charging of the photosensitive member becomes nonuniform. In order to obviate this problem, a method was employed that the screen grid is stretched by a spring, etc. However, this method includes such inconveniences that the number of the components is increased and the assembly operation becomes troublesome.